Pixel-by-pixel and patch-by-patch based methods simultaneously made their first forays into style transfer in SIGGRAPH 2001. Efros and Freeman [1]} proposed extending their patch based method by augmenting a 'content' and 'style' images (referred to a 'target image' and 'source texture' in their work) with correspondence maps that were relatively invariant to style and made it easier to match patches of texture onto sensible areas of content (they used a blurred luminance channel as their correspondence map, but also suggested using other derived features such as local orientation angles). Data-driven image modification was the primary focus of Hertzmann et al. [2]}, in which the framework of 'image analogies' was proposed. In essence the approach of 'image analogies' is quite similar to that of [1]}, and suggests improving the matches between a 'target image' and 'source texture' by providing an 'unfiltered' version of the 'source texture' which provides a better feature space for finding matches with the 'target image'. In the ideal case an 'unfiltered source texture' might be a photograph, and the 'source texture' would be a painting or drawing of the photograph's contents (with the exact same viewpoint, lighting, etc.). They demonstrated that this approach allowed the core algorithms of texture synthesis to be applied many image processing tasks, including colorization, super-resolution, and style transfer.
